1. Field of the Invention
The invention relates to glass substrates coated with light transmitting silver coatings, and to the production and processing of such silver coated glass substrates, useful in both architectural glazings and in vehicle windows.
2. Description of the Prior Art
It is known that transparent glass substrates with a thin silver coating, typically 5 nm to 30 nm thick, may be produced with a high light transmission and low emissivity i.e. which reflect a high proportion of infra-red radiation incident upon them but allow visible radiation to pass through. The use of such coatings on window glass (or plastics used in glazings) leads to a reduction in heat loss and results in valuable savings in heating costs. For optimum light transmission, the silver layers are sandwiched between thin anti-reflection layers of metal oxide. Such low emissivity coatings including a thin layer of silver sandwiched between layers of metal oxide are described, for example, in European patent specification EP No. 0 035 906 and UK patent specification GB No. 2 129 831.
According to European patent specification EP No. 0 035 906, a thin layer of material selected from the group consisting of titanium, zirconium, silicon, indium, carbon, cobalt and nickel is deposited between the silver and the overlying metal oxide layer to improve the long term durability of the coating. This additional thin layer has a thickness in the range 0.3 nm to 10 nm, preferably 1 nm to 5 nm. In each case, the thickness is selected to be sufficient to improve the durability of the coating, but not so great as to cause an unacceptable reduction in the light transmission of the coated product. The specification teaches that the coated substrate should preferably have a light transmission of at least 60% although the specification does include some Examples of coated substrates which have a light transmission of less than 60%; most of these are comparative Examples, but two, Example 56 (light transmission 58%) and Example 58 (light transmission 56%) are designated Examples of the invention. Their low light transmissions are due, in part, to the absence of an anti-reflection metal oxide between the silver layer and the glass (Example 56) or over the silver layer (Example 58).
In all the Examples, the coatings are on plastics substrates. UK patent specification GB No. 2 129 831 is concerned with problems which arose when attempts were made to apply the metal oxide layer overlying the silver layer by a reactive sputtering process in the presence of oxygen. Under these conditions, the low emissivity properties of the silver layer were lost, and the light transmission of the product was substantially lower than expected. These problems were overcome, according to patent specification GB No. 2 129 831, by sputtering an additional metal or metals other than silver in an amount equivalent to a layer 0.5 nm to 10 nm thick onto the silver layer.
U.K. patent specification GB No. 2 129 831 recommends the use of additional metal in an amount just sufficient to achieve the required low emissivity while obtaining a coating of the maximum possible light transmission. Unfortunately, coated glass produced according to UK patent specification GB No. 2 129 831 is not stable to heating in air, and the coating loses its properties of low emissivity and high light transmission when the coated glass is subjected to a thermal cycle required for bending or toughening the glass. Thus, in order to obtain a toughened or bent glass substrate bearing a silver coating and having high light transmission, it has been necessary to bend and/or toughen the glass substrate first, and then to apply the silver coating to the bent and/or toughened glass.